Robert J. Chase
English 9 Comprehensive
Mr. Derry
The temperature around the world is rising. The price for gasoline and petroleum is rising, causing your electric bill to cost you a small fortune. Looking for a better alternative? There is one that has been used for thousands of years. Though solar technology still needs some developing it is the clear alternative for the future.
“Solar energy is the worlds most abundant source of energy”(Kreith 38). In a world where scientists are predicting that fossil fuels will run out, it’s a good thing to know that there is one source of power that we can be sure will be around for many years to come.
The sun has been admired since the beginning of time though; finding ways of using the sun has been slow in the making. Modern Solar Technology has been in the making for over 2,400 years. The first civilization to try to use the sun’s rays was the Greeks. The Greeks designed their buildings to use the sun’s radiation to heat the air inside. Without the Greeks pioneering into using the sun’s power we may not have developed the solar technology we now have.
At the outer layer of the Earth’s surface there is what scientists call the “solar constant.” The solar constant is the amount of energy obtainable from the sun at the surface of the Earth’s atmosphere. The Solar constant is equal to 1.37 x 106 ergs, per second, per square centimeter (Thorsen 1). That’s quite a bit of energy, though on the Earth’s surface the amount of energy is much less because the atmosphere absorbs over thirty percent of the suns solar radiation (4). Also one reason that solar radiation is less at the earth’s surface is dependent upon the day of the year, the time of day, and latitude. The reason that the amount of energy that can be obtained at the Earth’s surface is affected by the day of year, time of day, and latitude because depending where you are the intensity of the suns rays can be very different.
In collecting solar energy there are two types of collectors used: flat plate and concentrating collectors (Thorsen 7). Flat plate collectors are the simpler of the two. As the name implies flat plate collectors are flat. These collectors collect the sun's rays, which heat the black absorber plates. The absorber pates in turn heat the carrier fluid. The absorber plate heats the carrier fluid because the fluid goes through tubes carved into the absorber plate. The carrier fluid, or the fluid that carries the energy, can either be a gas or a liquid (10).
When installing flat plate collectors figuring out how to orient them depends on a few variables. One variable in orienting the panels is the hemisphere they are in. for example in the Northern Hemisphere the panels must face towards true south to collect as much of the sun’s rays as possible. Another variable in orienting flat plate collectors is the latitude of the collection point (9). Most flat plate collectors that are placed on residential homes are fixed in the appropriate position, meaning that they do not move.
In a flat plate water heating system, after the absorber plates heat the carrier fluid, it then flows through pipes toward a small holding tank. The small holding tank, now containing a hot fluid, heats another tank full of water. That then goes to your faucet. This type of solar energy can be used just about anywhere that receives radiation, or light, from the sun.
“Concentrating collectors are devices that optically reflected and focus incident solar energy onto a small receiving area” (11). A concentrating collector is quite different than a flat plate collector. Concentrating collectors can come in many different sizes. The main principle of all concentrating collectors is that all of the radiation from the sun that enters the collector is sent to the same point, the focal point. Using this principle the energy that enters the collector is magnified. In a normal concentrating collector the temperatures can range from one hundred degrees Celsius to thousands of degrees Celsius. This type of collector has many applications, such as: air conditioning, and central power generation (Thorsen11).
One problem with solar collectors is that in any one location on Earth you cannot collect energy from the sun all the time. One reason you can not collect the sun’s radiation all the time is because of the Earth’s rotation, which causes the days and nights. Another reason that solar radiation can not be collected all the time is because of weather. When it’s cloudy the sun’s radiation can not heat the solar collectors adequately enough to heat the carrier fluid.
Another type of solar system that uses basic concept of flat plate collectors is photovoltaics. Photovoltaics is the process of turning the sun’s radiation directly into electrical energy (7). Photo means light, and voltaic means electricity. When you put the two together, photovoltaics, they literally mean getting energy from the sun (Aldous 4).
The development of Photovoltaics started around 1876. This was when Europeans and the North Americans where laying the lines for the telegraph, scientists started using chemicals as a testing device. During one such test the scientists noticed that the chemical being used, Selenium, created an electrical current when light was shown upon it (Perlin 15). This discovery led to the findings of other chemicals that do similar or the same thing. By the 1950’s, scientists were using silicon in photovoltaic cells to make them more efficient (17). The reason for developing photovoltaic cells was because NASA needed them for their space shuttles and satellites (19).
When sunlight hits a photovoltaic (PV) cell an important reaction takes place. The radiation hits the semiconductor, most often silicon; the radiation is then taken in (Aldous 5). Each photon, or particle of light, with the right amount of energy in it, will bump into an electron sending it away from the silicon atom, while the photon takes its place as the electron. After the photon knocks the electron away from the atom, that electron becomes what we call a free-electron. The free-electrons then travel through a pathway. The electrons flowing through the pathway is called the current. In a PV cell there is a magnetic field which makes the free-electrons flow through the pathways. This is called the voltage. You find a PV cells power by adding the voltage and the current together (Aldous 16). To produce a sensible amount of power you would make a module, or network, of PV cells.
When thinking about PV cells you must remember that not everything is one hundred percent efficient. Most photovoltaic cells are only able to obtain twenty-five percent of the radiation that reaches it through the atmosphere (18). One reason for the loss of energy is that PV cells can only absorb a certain wavelength of photons. If the radiation reaching the cell has photons of the wrong wavelength they go right through the cell and don’t cause any reactions. The amount of energy needed to knock an electron off a silicon atom is called the band gap energy. If a photon has the correct wavelength but has more energy than needed for the band gap then the extra energy will be lost. Photons with too much energy, or of the wrong wavelength make up seventy percent of the total energy loss in a PV cell (19). The metal pathway for conducting the free-electrons around the cell can cause a loss of energy (21). Silicon, which is a semiconductor, has a higher resistance than metal. Resistance inside a photovoltaic cell, or the series resistance, is very high meaning there is a great amount of energy loss (21).
Though there are a multitude of hindrances in a PV cell that cause energy loss there are ways to prevent potential losses. Because Silicon is a reflective material, scientists have put antireflective coating on it, which reduces that energy loss by 5% (22). Another development used to reduce energy loss is a metal contact grid to shorten the distance the free-electrons need to travel to leave the cell, therefore making the cell more efficient. Most PV cells are around 25-30% efficient (Thorsen 15). This means that about 70-75% of the potential energy is lost. Though there is substantial amount of energy loss there is still quite a bit of energy being collected successfully and can be used in most buildings.
If you’re looking to put a PV system on your house you have to take in a number of variables before purchasing your unit. First, do you have an area, such as a roof, that faces in the correct direction to get direct sunlight for a large portion of the day? You also need to find meteorological data from a web site that tells you the amount of energy that a solar system would be able to produce in a year, taking into consideration the different weather patterns of the seasons (17). This data is essential because the amount of solar equipment you need to put on your house does not depend on the size of the house. It is dependent on the amount of energy you use and the amount of energy you can collect in your area (“How Do I” 1).
The fact that the sun does not shine all the time in any one place on Earth presents a problem. For this problem there are a couple of routes that are cost efficient. One self-sufficient way of having power through the night is to have batteries connected to the photovoltaic system and during periods of low demand the system will charge the batteries for future use. These batteries systems are fairly simple and over time can save someone a bunch of money because you only have to pay once for the system, and you don’t have to buy any fossil fuel for it. There is another way of securing power for times that there is no radiation available from the sun. It is called a grid tied system. This means you are connected to the local power plant but you only take power from the grid when your PV system cannot produce enough. These are just two examples for having power during periods of low production (Thorsen 17).
Though solar is still in a developing state, it is the most versatile and promising source of energy for the future. Solar has many forms; it can be used in a flat plate collector to create heat or an electrical current, it can also come in the form of a concentrating collector for air conditioning and central power generation. If you want to make an attempt at saving the Earth, solar is the way to go.
Works Consulted
Aldous, Scott. “How Solar Cells Work.” http://www.howstuffworks.com/solar-cell.htm/printable. (May 16, 2002)
“How do I add Solar to my home?” http://www.mrsolar.com. (May 7, 2002)
Kreith, Frank. Principles of Solar Engineering. New York, NY: Hemisphere, 1978.
Perlin, John. “History of Solar Energy.” http://www.abc.net.au/rn/science/earth/stories/s225110.htm. (May 20, 2002)
Thorsen, Richard S. “Solar Energy.” Encarta Encyclopedia. [CD-ROM] Microsoft, 1999.